The present invention refers to a system and to a method for the measurement of the relative position of an object with respect to a predetermined point of reference.
An exemplary application of the system and of the method according to the present invention is that of the measurement of the relative position of a ball with respect to a specific line of a field of play.
In several sports, a referee acknowledges an event depending on the relative position of a ball with respect to a specific line of the field of play. e.g., in Soccer a referee awards a score to a team only when the ball has crossed over the goal line.
However, the referee can autonomously note the scoring of a goal solely when the latter is apparent, e.g. due to the swelling out of the netting or to the ball remaining inside of the goal. However, quite frequently the ball crosses over the goal plane and, without touching the net, immediately exits therefrom due to an odd rebound onto the soccer held, produced by contact with the goal posts, or a player's clearance.
When the ball speed is not overly high (it could even reach the 120 Km/h) the referee, in order to decide on the event at issue, can be aided by assistants, who however should have ideal observing conditions, i.e. be positioned on the goal line having the goal in sight. Otherwise the referee, having to decide one way or the other, runs the risk of awarding a non-existent (phantom) scored goal, or of not awarding an actually scored goal.
To solve this problem, several known systems, all referable to the same category, provide the use of sensors, inserted inside of the goal structure, receiving a signal from transmitters applied into the playing ball when the latter crosses the goal mouth.
However, such systems entail the remarkable disadvantage of being invasive, requiring electronic devices to be inserted in the playing structures (goals and ball). Hence, in order to use the former a general modification to the fields of play and the use of specific balls would be required. These devices are not always applicable, as the modification required to the playing structures could interfere with the laws of the game.
Moreover, a known visual-type system relies instead on the observation of the field of play by suitably positioned cameras. This system determines the position of the vertical projection of the center of mass of the ball onto the plane of the field of play, exploiting the information (known a priori) on the dimensions of the various areas thereof.
However, this system entails the disadvantage of exclusively detecting the ball crossing over a determined line, yet providing no indication about the height above ground of the ball during its crossing, an information crucial in order to confidently claim that the ball has crossed over the goal plane.
A first system for determining the position of an object in a space is known by WO 00/72264. Such system provides for the identification of the object into the images by means of the so-called chroma-key system that operates by comparing areas of the images with a treshold.
Another system is disclosed in EP 0662600. In such case a system is provided for determining the absolute spatial position of an object.
Nevertheless, such system has several drawbacks. In particular the system is only capable of determining the position of an object that moves on a single plan, in a predetermined direction.
Furthermore, the recognition of the object into the acquired images is simply performed by comparison with fixed tresholds, making the result dependent from the environmental conditions.
Purpose of the present invention is to solve the abovecited problems of the known art providing a method for the measurement of the relative position of an object with respect to a point of reference, comprising the following steps:                acquiring a plurality of images, each of said images being apt to display said point of reference;        processing each image of said plurality of acquired images; and        computing said relative position of said object with respect to said point of reference,        
characterised in that said step of processing each image further comprises a step of recognizing said object inside each of said images, said step of recognizing said object being performed by a classifier.
The present invention further provides a system for the measurement of the relative position of an object with respect to a point of reference comprising:                one or more image acquisition subsystems, each subsystem being apt to acquire a plurality of images, each of said images displaying said point of reference; and        a unit for the processing of said acquired images,        
characterised in that said processing unit comprises a classifier for recognising said object inside each of said image.
Hence, a field of play comprising such system could advantageously be provided.
For simplicity's sake, hereinafter reference will still be made to the application of the system in the case of Soccer. Of course, it is umderstood that the described system and method could be useful in any other application entailing the same technical problem.
The main advantage of the method and of the system according to the present invention lies in that those entail no modification to any component of the field of play, or to the ball.
A second advantage lies in the robustness and in the reliability of the detection of the scored goal event. Said method integrates a geometrical measurement of the position of the ball in the three-dimensional space (obtained by a binocular system) to a qualitative assessment of the observed event (obtained with a monocular system) thereby emulating the activity of a human observer enjoying the best observing conditions. The integration of the measurements provided by said systems ensures a high precision rate and a minimum error probability in any situation, including those of partial ball obstruction, e.g. by one or more players.
A third advantage lies in the completeness of the information on the position of the object with respect to a point of reference, as the method provides the three-dimensional coordinates thereof.
A fourth advantage lies in that the system according to the present invention uses, for image acquisition, high-speed digital cameras, whose performances surpass those of the common cameras as well as those of the human eye.
A further advantage lies in that the system according to the present invention provides an objective digital recording of the event itself, obtained by an advantageous positioning of the cameras and the concomitant acquisition of the observed scene. This recording enable to subsequently review the scene at will, to validate the signaled event (goal/non-goal), as it typically happens in the case of a digital viewer having high time resolution and simultaneous multivision.